Why do Americans call single beds 'twin beds'?

Twin bed is either of a pair of matching single beds ready to be joined to another twin bed to create twin beds. The word simply describes a bed which is stackable with another bed. A single bed isn't stackable, at least not the way you'd want it to.

UPDATE according to what John Y commented:

Well, wikipedia says:

U.S. terminology refers to a twin bed to mean what is known as a single bed in other countries, whereas other countries understand twin beds to be two single beds in the same room.

While wiktionary says:

twin bed (plural twin beds): Either of a pair of matching single beds.

Oxford dictionary says:

one of a pair of matching single beds, particularly in a hotel or guest room intended for two people.

So to sum it up, and now correct me if I'm wrong:

1 bed (US): twin bed
2 beds (US): twin beds

1 bed (UK): single bed or a twin bed if it belongs to a pair of beds
2 beds (UK): twin bed or twin beds

Therefore twin bed is ambiguous, unless the dictionaries are wrong.

Small cot-sized beds are often found in pairs: in barracks, dormitories, prisons, hospitals, children's rooms, and other places. Perhaps such beds were frequently referred to as "twin beds" formally or informally at the time that bed sizes were being standardized, and furniture manufacturers just decided to keep the name that everyone had been using for that size of bed anyway.

So why are they called "twin beds" even when they don't appear in pairs? It may be something of a backformation: if two such beds are twin beds, obviously one such bed must be a twin bed.

Double or Full is larger than a twin (single) and smaller than a queen. Double means two people (double) can sleep on it if need be and the term FULL I think is because it is a full sized bed, not a kids single bed, or twin.... lol 2 twin beds side by side IS a King Sized bed. I think the term TWIN refers to the variations in which it can be used... it has a "twin" to make a King, a "twin" to make bunk beds, so if a child is part of a twin set..... Identical, etc., THAT person would refer to themselves as a twin.

In the 1950s, a single was as wide as the average person's shoulders. A twin was a single & a half. A double was two singles. A queen was a single & a twin. A king was 2 twins (or 3 singles). Nowadays, the single is forgotten, so everyone is confused. When I was a child in the 1950's, my bed was a twin and you could put two first graders in it side by side. My best friend had a single bed, so when I slept over at her house, I slept in her bed and she slept on the floor.

I sell beds in the UK, and the sizes that we have are small single bed which is 2' 6" wide and 6' 3" long; we have large singles which are 3 feet wide and 6'3" long; and these beds can all be joined together with what's called zip and link, where the bases bolt together and the mattresses zip together. We have a small double, 4 ft wide and the same length as singles; a double bed which is 4' 6" wide, same length; a king size bed which is 5ft wide and 6' 6" long; and a super king which is 2 x 3' 6" beds that zip and link together, but you have to have custom made bedding fit the super king. That's the UK system.